Embossing is one of the operations that are normally performed on plies or sheets of tissue paper, to produce paper articles for personal and household cleaning and hygiene, or also for commercial and industrial use, such as toilet paper, kitchen towels, paper napkins and handkerchiefs and the like.
Embossing is an operation that is performed for the dual purpose of improving aesthetic properties and increasing functional properties, in particular the softness, smoothness, absorption capacity or thickness of the finished material.
Normally, a tissue paper material, such as kitchen towel or toilet paper, is produced from two or more plies of paper embossed separately and subsequently bonded together by applying a glue and laminating the plies between counter-rotating rollers defining a lamination nip.
Embossing is typically performed by feeding each ply between an embossing roller, equipped with protuberances, and a pressure roller with an outer surface coated in yielding material, typically rubber. In this case this is known as steel to rubber embossing, as the embossing roller is typically made of steel. In some cases embossing is performed between two rollers made of steel or another hard material, one equipped with protuberances and the other with corresponding recesses. The protuberances of the embossing roller produce corresponding protuberances or projections in the paper ply. The protuberances formed in the two outermost plies are facing the inside of the finished product.
According to a possible technique (known as tip-to-tip”), the two plies of paper web material are bonded by making the protuberances of one ply coincide with the protuberances of the other ply, having previously applied glue to the protuberances of one of the two plies, or at least to some of said protuberances. In practice, two embossing rollers which emboss the two paper plies separately by means of respective pressure rollers, form therebetween a lamination nip, through which the two embossed plies are fed before being detached from said rollers. In the lamination nip the protuberances of one roller coincide with the protuberances of the other roller and the reciprocal distance between the rollers is such as to cause localized compression of the plies at said protuberances.
A tip-to-tip embossing unit for obtaining a product of this type is described in U.S. Pat. No. 3,414,459. The tip-to-tip embossing technique has gradually been perfected and improved, in order to solve particular problems arising with this processing method. U.S. Pat. No. 5,096,527 describes, for example, a technique to reduce the vibrations and wear in tip-to-tip embossing units. U.S. Pat. No. 6,113,723 describes a distribution of protuberances having the object of increasing the adhesion strength through a particular arrangement of the protuberances. U.S. Pat. No. 5,736,223 describes a method for producing a paper article in sheet form comprising three layers or plies of tissue paper.
U.S. Pat. No. 5,173,351; U.S. Pat. No. 6,032,712; U.S. Pat. No. 6,245,414; U.S. Pat. No. 6,053,232 describe embossing-laminating units which with particular measures prevent concentrated wear of the protuberances also when these do not completely coincide, but give rise to a partial correspondence in areas, between some of the protuberances of one roller and some of the protuberances of the other. U.S. Pat. No. 3,961,119 describes a tip-to-tip embossing unit, wherein two mutually cooperating embossing rollers are equipped with helical projections. The projections of one embossing roller present protrusions that mesh with recesses provided in the helical projections of the opposed embossing roller.
According to a different technique, the two plies are embossed separately, each between an embossing roller and a counter-roller or pressure roller. The two plies are then mutually joined so that the protuberances of one ply are nested between the protuberances of the other ply. This is known as “nested” embossing. Lamination of the two plies is obtained between one of the embossing rollers and a laminating roller, while the two embossing rollers do not touch. Examples of embossing-laminating devices of this type are described in U.S. Pat. No. 3,556,907; U.S. Pat. No. 3,867,225; U.S. Pat. No. 5,339,730.
U.S. Pat. No. 5,686,168 describes a method of nested embossing, wherein the plies are bonded by lamination between two opposed embossing rollers.
U.S. Pat. No. 6,578,617 and U.S. Pat. No. 6,470,945 describe embossing units that can perform embossing according to both the aforesaid techniques. To switch from tip-to-tip to nested embossing, the angular phase and/or the axial position of the two embossing rollers can be modified.
U.S. Pat. No. 6,261,666 describes a similar device for alternatively performing tip-to-tip or nested embossing. Another similar device is described in U.S. Pat. No. 6,109,326.
Initially, embossing was performed according to very simple geometric patterns, with uniform distribution of truncated-cone or truncated-pyramid shaped protuberances. This embossing had a prevalently technical function, used to produce a product of adequate thickness and with sufficient softness and absorption capacity.
Currently, embossing must increasingly achieve a dual function, both technical-functional and aesthetic. Therefore, embossing patterns and embossing devices have been developed which make it possible to obtain a product that is aesthetically pleasing and at the same time suitable to satisfy the aesthetic requirements and increasing commercial requirements demanded of these products. Embossing is no longer produced only through simple geometrical patterns, but by combinations of embossing areas of greater or lesser density (at times of micro-embossing) and of decorative patterns embossed and optionally printed. Examples of complex embossing patterns are described in U.S. Pat. No. 6,136,413; U.S. Pat. No. 5,846,636; U.S. Pat. No. 6,106,928.
A new embossing technique that makes it possible to obtain particularly refined and easily interchangeable motifs is described in U.S. Pat. No. 6,755,928 and in U.S. Pat. No. 6,681,826.
WO-A-2006/027809 describes a convertible embossing unit, wherein a pair of fixed sides cooperate with a pair of oscillating sides to define two pairs of supporting seats for the embossing rollers. In order to arrange the embossing rollers in two alternative positions, the mobile sides can cooperate directly with the fixed sides, or alternatively the fixed and mobile sides can be equipped with elements to close and complete the seats to support the ends of the embossing rollers. The embossing unit thus configured, although characterized by the possibility of modifying the arrangement of the rollers, has limited flexibility.
One of the requirements considered most necessary in the paper converting industry is the possibility to adapt production lines to different market demands, with fast and simple modifications of the line, to avoid stoppage thereof and consequent loss of production.